Abstract
Desiccation tolerance appeared as the key adaptation feature of photoautotrophic organisms for survival in terrestrial habitats. During the further evolution, vascular plants developed complex anatomy structures and molecular mechanisms to maintain the hydrated state of cell environment and sustain dehydration. However, the role of the genes encoding the mechanisms behind this adaptive feature of terrestrial plants changed with their evolution. Thus, in higher vascular plants it is restricted to protection of spores, seeds and pollen from dehydration, whereas the mature vegetative stages became sensitive to desiccation. During maturation, orthodox seeds lose up to 95% of water and successfully enter dormancy. This feature allows seeds maintaining their viability even under strongly fluctuating environmental conditions. The mechanisms behind the desiccation tolerance are activated at the late seed maturation stage and are associated with the accumulation of late embryogenesis abundant (LEA) proteins, small heat shock proteins (sHSP), non-reducing oligosaccharides, and antioxidants of different chemical nature. The main regulators of maturation and desiccation tolerance are abscisic acid and protein DOG1, which control the network of transcription factors, represented by LEC1, LEC2, FUS3, ABI3, ABI5, AGL67, PLATZ1, PLATZ2. This network is complemented by epigenetic regulation of gene expression via methylation of DNA, post-translational modifications of histones and chromatin remodeling. These fine regulatory mechanisms allow orthodox seeds maintaining desiccation tolerance during the whole period of germination up to the stage of radicle protrusion. This time point, in which seeds lose desiccation tolerance, is critical for the whole process of seed development.
Highlights
The ability to sustain dehydration and survive during desiccation appeared as a crucial evolutionary step, which allowed the first plants colonizing the terrestrial habitat [1]
Early- and middle seed maturation stages are accompanied with accumulation of nutrients
At the late maturation stage, orthodox seeds develop desiccation tolerance, which allows maintaining seed viability after the loss of up to 95% of water and onset of dormancy
Summary
The ability to sustain dehydration and survive during desiccation appeared as a crucial evolutionary step, which allowed the first plants colonizing the terrestrial habitat [1]. The role of the genes, which encoded adaptation of the whole organism to water loss at earlier steps of plant evolution, in higher vascular plants is restricted to the protection of spores, seeds and pollen from dehydration. Some authors believe that the plant species with recalcitrant seeds evolved from ancestors with orthodox seeds as a result of adaptation to wet climate with constant rains, where immediate germination upon separation from the parental plant would be evolutionary advantageous [1] Besides these two contrasting seed development strategies, so-called “intermediate” plant species, the mature seeds of which are able to sustain slight dehydration, are known [1,5,6]. It is assumed that the molecular mechanisms underlying desiccation tolerance of resurrection plants and mature orthodox seeds are essentially similar [4]
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